1,4-bis-(cycloalkylthio)piperazines

ABSTRACT

Bis(cycloalkylthio)piperazines are prepared which are excellent inhibitors of premature-vulcanization.

= atent Col-an et al. 1 Aug. 1, 1972 1,4-BIS- [56] References Cited (CYCLOALKYLTHKDPIPERAZINES UNITED STATES PATENTS 72 Inventors: Aubert Y. Coranuse b E. Ke 1 wood; Chester f z Jr i 2,514,181 7/1950 Smith ..260/268 X ofAkron Ohio 2,955,104 10/1960 Smith ..260/268 X V r 3,449,323 6/1969 Dunbar ..260/268 X Asslgnw Monsanto p y, 3,513,139 5/1970 Coran ..260/268 x Filed: March 9, Damico R [21] APPL 17,971 FOREIGN PATENTS OR APPLICATIONS Related us. Application Data 1,123,580 12/1968 Great Britain ..260/268 S [63] Continuation-impart of Ser. No. 518,987, Jan. Primary Examiner-Donald G. Daus 6, 1966, Pat. No. 3,513,139. Attorney-Richard O. Zerbe, J. E. Maurer and Neal E.

Willis [52] US. Cl ..260/268 S, 260/79.5 C, 260/239 BC,

260/247.1, 260/306.6 A, 260/326 SS, [57] ABSTRACT I l 3 24 38 Bis(cycloalkylthio)piperazines are prepared which are 8 S 239 4 excellent inhibitors of premature-vulcanization.

2 Claims, No Drawings 1 ,4-BIS-(CYCLOALKYLTHIO)PIPERAZINES This application is a continuation-in-part of application Ser. No. 518,987 filed Jan. 6, 1966, now US Pat. No. 3,513,139, May 19, 1970.

This invention relates to an improved vulcanizing process for rubber and to the stocks obtained by using this improved process. The invention relates to improved accelerator-inhibitor combinations for rubber. More particularly, the invention relates to a method for preventing the aging and premature vulcanization of rubber stocks and to the rubber compositions obtained by using this method. The invention also relates to 1,4- bis(cycloalkylthio)piperazines useful in the improved process.

An object of this invention is to provide a method to effectively prevent the premature vulcanization of rubber. A further object of this invention is to provide a method to increase the available processing time prior to the actual vulcanization of rubber. A further object of this invention is to provide an increased cure rate for rubber. A further object of this invention is to provide a method to prevent the premature vulcanization of gum rubber in storage containing a vulcanization and accelerating agent. A further object of this invention is to improve the storage stability of an uncured rubber mixture containing an accelerator. A further object of this invention is to provide a method to prevent the premature vulcanization of rubber during the actual vulcanization step. A further object of this invention is to prevent the premature vulcanization of rubber at any time. A further object of this invention is to provide a safer method for processing and vulcanizing rubber. Other objects of the invention will become apparent as the description proceeds. These objects are accomplished by using the accelerator-inhibitor combinations of this invention.

SUMMARY OF THE INVENTION We have discovered a class of sulfenamides which are outstanding inhibitors of premature vulcanization. The characteristic nucleus is wherein x and x taken together with the nitrogen atom is R is cycloalkyl. The term cycloalkyl includes cycloalkyl radicals of five to 12 carbon atoms, preferably five to eight.

Aryl sulfenamides derived from secondary amines have been described as vulcanization-accelerating agents. See US. Pat. Re 19,286, Zaucker assigned to l. G. Farbenindustrie (1934). It has also been reported that certain aryl sulfenamides characterized by the presence of an electronegative group in the aryl nucleus which ordinarily alone have no substantial accelerating power will, nevertheless, retard the action of nitrogen-containing vulcanization accelerators and will increase their strength. See US. Pat. Nos. 2,382,813 1944) and 2,460,393 1945) to Paul assigned to United States Rubber Co. Smith reported that 1,4-bisphenylthio piperazine was a curing agent for fluoroelastomers. See U.S. Pat. No. 2,955,104, Smith assigned to E. I. duPont de Nemours (1960). More recently, Lober reported that pentachlorophenyl sulfenamides were vulcanization accelerators of the delayed-action type. See US. Pat. No. 3,047,546, Lober assigned to F arbenfabriken Bayer 1962).

The stocks of this invention may contain an amine to protect the rubber from degradation. The amine usually reduces processing safety, but the new vulcanizing systems are effective to prevent premature vulcanization in the presence thereof. The data infra illustrate that a combination of an accelerator and an inhibitor of this invention is an improved rubber additive which allows significantly longer and safer processing time for rubber than that provided hitherto for the rubber industry. Further, the accelerator-inhibitor combinations of this invention may improve the modulus of a vulcanizate considerably. The storage stability of a rubber mixture containing an accelerator-inhibitor combination of this invention is improved compared to a rubber mixture containing an accelerator alone. Inhibitors of this invention also enhance the antidegradant properties of a vulcanizate.

Examples of the premature-vulcanization inhibitors of this invention are l,4-bis(cyclopentylthio)piperazine, 1,4-bis(cyclohexylthio)piperazine, l ,4-bis( methylcyclohexylthio)piperazine, l ,4- bis(cyclooctylthio)piperazine, l ,4- bis( cyclododecylthio)piperazine, l ,4-bis(ethylcyclohexylthio)piperazine and 1,4-bis(dimethyl cyclohexylthio)piperazine.

Rubber stocks containing delayed-action uccclcrutors can be used in the process of this invention. Cheaper, more scorchy accelerators can also be used with an excellent degree of improvement. The improved vulcanizing method of this invention can be used advantageously to process stocks containing furnace blacks as well as stocks containing other types of blacks and fillers used in rubber compounding. The invention is also applicable to gum stocks.

Our invention is applicable to rubber mixes containing sulfur-vulcanizing agents, organic accelerators, and antidegradants. For the purposes of this invention, sulfur-vulcanizing agent means elemental sulfur or sulfurcontaining vulcanizing agent, for example, an amine disulfide, or a polymeric polysulfide. The invention is applicable to vulcanization accelerators of various classes. For example, rubber mixes containing the aromatic thiazole accelerators which include N-cyclohexyl benzothiazole-2-sulfenamide, Z-mercaptobenzothiazole, N-tert-butyl benzothiazole-2-sulfenamide, 2-benzothiazolyl diethyldithiocarbamate, and 2- (morpholinothio)benzothiazole can be used. Thiazole accelerators other than aromatic can be used. Stocks containing accelerators, for example tetramethylthiuram disulfide, tetramethylthiuram monosulfide, a1- dehyde amine condensation products, and guanidine derivatives, are substantially improved using the method of our invention. Stocks containing mixtures of accelerators are substantially improved by using the method of our invention. Rubber mixes containing antidegradants, for example N-1,3-dimethylbutyl-N- phenyl-p-phenylenediamine, N,N'-bis( 1,4-dimethylpentyl)-p-pheny1enediamine, N,N-bis(2-octy1)-pphenylenediamine, and other phenylenediamines, ketone, ether, and hydroxy antidegradants and mixtures thereof, are substantially improved using the process of our invention.

The inhibitors of our invention can be used in natural and synthetic rubbers. Synthetic rubbers that can be improved by the process of this invention include cis-4- polybutadiene, butyl rubber, ethylene-propylene terpolymers, polymers of 1,3-butadiene, for example 1,3- butadiene itself and of isoprene and copolymers of 1,3- butadiene with other monomers, for example styrene, acrylonitrile, isobutylene, and methyl methacrylate. The invention relates to diene rubbers, and the terms rubber and diene rubber are synonymous for the purposes of this invention.

The following tables illustrate the invention in greater detail and the best mode for carrying it out, but are not to be construed as to narrow the scope of our invention. For the rubber stocks tested and described, infra, as illustrative of the invention, Mooney scorch times at 121 C. and 135 C. are determined by means of a Mooney plastometer. The time in minutes required for the Mooney reading to rise 5 points above the minimum viscosity is recorded. Longer times are indicative of the activity of the inhibitor. Longer times on the Mooney scorch test are desirable because this indicates greater processing safety. Percentage increases in scorch delay are calculated by dividing the Mooney scorch time of the stock containing the premature-vulcanization inhibitor by the Mooney scorch time of the control stock, multiplying by 100, and subtracting 100. These increases show the percentage improvement in scorch delay over the control stock which contains no inhibitor. Curing characteristics are determined by means of the Monsanto Oscillating Disc Rheometer described by Decker, Wise, and Guerry in Rubber World, Dec., 1962, p. 68. From the Rheometer data, R. M. T. is the maximum modulus torque in Rheometer units, or t is the time in minutes for a rise of 3 or 2 Rheometer units, respectively, above the minimum reading and is the time required to obtain a modulus 90 percent of the maximum. The difference between these two times, t -t is indicative of cure rate. The specific rate constant k is measured in reciprocal minutes. See Coran, 37 Rubber Chemistry and Technology 689 (1964). Higher values of k indicate higher rates of cross linking. With the aid of k k can be calculated and provides another measure of processing safety. Lower values of k indicate less scorch or a longer delay time.

The trademarks of some compounds used in the practice of this invention are Santocure, Santocure MOR, Santoflex 77, Santocure NS and Thiofide. San tocure is the accelerator N-cyclohexyl benzothiazole-Z- sulfenamide. Santocure MOR is the accelerator 2- (morpholinothio)-benzothiazole. Santoflex 77 is the antidegradant N,N-bis( 1,4-dimthylpentyl)-p-phenylenediamine. Santocure NS is the accelerator N-tertbutyl benzothiazole-Z-sulfenamide. DPG is an accelerator reported to be diphenylguanidine. Thiofide is an accelerator reported to be benzothiazyl disulfide.

PREFERRED EMBODIMENTS Table 1 illustrates the use of l,4-bis(phenylthio)piperazine as a premature-vulcanization inhibitor in a natural rubber masterbatch containing the following:

Parts per hundred rubber Natural rubber smoked sheets 100 The natural rubber A-6 masterbatch is comprised of the following:

Parts per hundred rubber Natural rubber smoked sheets 100 High abrasion furnace carbon black 50 Zinc oxide 5 Stearic acid 3 Hydrocarbon softener 10 TABLE 11 Stocks 1 2 A-fi Masterbatch 168 168 Sulfur 2.5 2.5 N-1,3-dimethylbuty1-N'-pheny1- 2.0 2.0 p-phenylenediamine Santocure MOR 0.5 0.5 1 ,4bis(n-buty1thio)piperazine 1 .0 Mooney scorch at 135C.

1,, 12.9 23.8 Increase in scorch delay Rheometer at 144C.

RMT 55.0 64.0 1, 9.8 17.1 27.0 32.8 k 0.138 0.145 1 -1, 17.2 15.1

TABLE III Stocks 1 2 A-6 Masterbatch 168 168 Sulfur 2.5 2.5 N- 1 ,3-dimethy1buty1-N-phenyl 2 .0 2.0 p-phenylenediamine Santocure MOR 0.5 0.5 1 ,4 bis( benzylthio )piperazine 1 .0 Mooney Search at 121C.

1 44.5 69.5 Increase in scorch delay 57 Increase in scorch delay 57 Rheometer at 144C.

RMT 54.0 64.1 9.8 18.7 0.145 0. 1 3 4 Table IV illustrates the use of 1,4-bis(cyclohexylthio)piperazines as premature-vulcanization inhibinitrogen compared to 61.09 percent carbon, 9.61 percent hydrogen, 20.38 percent sulfur and 8.90 percent nitrogen calculated for C 11 N The same procedure, with the exception tors in a natural rubber masterbatch containing the fol- 5 fifi gl l f r's h g lis iii z qz i flowing; y 0 omop1peraz1ne w 1c me a na ysis Pam per hundmdmbber gives 62.33 percent carbon, 9.85 percent hydrogen, Natural rubber smoked sheets 100 8.52 percent nitrogen and 19.37 percent sulfur comii g g zzi Pembrasm furnace 45 pared to 62.39 percent carbon, 9.82 percent hydrogen, zinc oxide 34) l8.5lllj i ercent niltosgen and 19.51 percent sulfur calcu- Stearic acid 2.0 at o Hydrocarbon Softener Table Qf ili usiraites the use of l,4-b1s(cyclohexylthio)piperazine and l,4-bis(phenylthio)piperazine as The i of each St0C 1S de erm ed y means o the premature vulcanization inhibitors in styrene-bu- Mooney plastometer m the manner d crib an he l 5 tadiene rubber masterbatch containing the following: percent increase in scorch delay of stocks 2 and 3 calculated as compared to control stock 1 and the percent t Pam per hundred rubber increase in scorch delay of stocks 5 and 6 calculated as Oil-mauled styrene-bundle rubber lSAF carbon black 65.0 compared to control stock 4. Zinc oxide Stearic acid 1.0 TABLE IV Hydrocarbon sofiener 1.5

TABLE V Stocks 1 2 3 4 5 6 Masterbatch 155 155155 155 155 155 1 Stock! 3 gum" NS Master-batch 208.0 208.0 208.0 antocure u ur 2.0 2.0 2.0 N-1,3 dimethylbutyl N- l ,3-dimethylbutyl-N'-phenyl- 2.0 2.0 2.0 -N'-pheny1-p-phenyl p-phenylenediamine enediamine 2.0 2.0 2.0 Santocure NS 1.0 1.0 1.0 l,4-bis(cyclohexyll,4-bis(pheny1thio)piperazine 0.5 thio)piperazine 0.5 0.25 0.5 1,4-bis(cyclohexylthio)piperazine 0.5 lfi kblilskyclohexyl- 0 5 Mooney Scorch at 135C. 23 7 36 l 43 2 I 10 01110 1 erazine l Mooney Sc oi' ch at 121C. zzhlncr ase in S]C503l'C(l:1 delay 52 82 Increase in eometer at scorch delay 196 91 I75 99 RMT 65.2 62.5 62.9

The sulfenamides derived from piperazine are Although the invention has been illustrated by typiprepared by the method disclosed by Smith, supra, cal examples, it is not limited thereto. Changes and which comprises reactin the appropriate lf l modifications of the examples of the invention herein chloride with piperazine. 1,4-Bis(cyclohexylthio)- 40 chosen for purposes of disclosure can be made which piperazine melts at 74 75 C A f rr d th d f do not const1tute departure from the sp1r1t and scope of preparing it comprises condensing N-cyclohexylthio thell1Vml0I1- phtha11m1de w1th p1peraz1ne as described in appl1cat1on The odlments of the invention 1n wh1ch an excluof J. E. Kerwood of even date Ser. No. 17,944. SW6 P p y Prlvllege 1S claimed are defined as Piperazine (8.6 g., 0.1 mole) dissolved in 75 m1. of lows! isopropanol is added to a solution of Ncyclohexylthio A P Ofthe formula phthalimide in 250 m1. of isopropanol at 55 C. The mixture is stirred for 6 hours at 55 C., then filtered to I( remove 29.5 grams of phthalimide. The filtrate is evaporated; the residue dissolved in heptane and filtered to remove residual impurities. The heptane is removed from the filtrate and 9.5 grams of N,N- dicyclohexylthio piperazine is recovered. Recrystalh R l alk l ff h b lized from methanol, the product melts at 74-75 C. w 2 g}: eye 0 d 3; f l fi F 1 f g l Ana1ys1s g1ves 61.08 percent carbon, 9.52 percent e compoun c w erem cyc 0 hydrogen, 20.32 percent sulfur and 8.93 percent 

2. The compound of claim 1 wherein R is cyclohexyl. 